1. Field of the Invention
This invention relates to an exhaust system for an outboard motor, and more particularly to an improved exhaust system that discharges exhaust gases to the atmosphere.
2. Description of Related Art
Typical outboard motors have engines atop thereof for powering propulsion devices and protective cowlings surround the engines. The engines are provided with exhaust systems that discharge exhaust gases.
A typical exhaust system includes a main exhaust route that discharges exhaust gases via a submerged main exhaust port to the body of water in which the outboard motor operates (usually through the propeller hub), and an idle exhaust route that discharges the gases to the atmosphere via an above-water idle discharge port. When the outboard motor runs at a normal speed, the exhaust gases are discharged through both the main and idle exhaust routes. When the outboard motor runs the watercraft at a relatively low speed or the engine is just idling, the exhaust gases are discharged only through the idle exhaust route due to backpressure at the submerged main exhaust port.
The exhaust gases generally hold large amounts of energy that manifest in exhaust noise. One of significant issue for outboard motor design involves how to treat the exhaust noise. As to the main exhaust route, the exhaust noise treatment is relatively easy because the motor can have a plurality of expansion chambers en route to a discharge port. Additionally, the water body itself silences the exhaust noise as energy is dissipated in the water. The noise treatment, however, is a serious problem with the idle exhaust route due in part to the fact that the route is normally contained in a relatively narrow space. Moreover, the idle route must discharge the gases directly to the atmosphere.
Expansion chambers have been previously used in the idle exhaust route to silence exhaust noise before discharge. For example, U.S. Pat. No. 4,963,110 discloses an exhaust system that has a detachable expansion chamber mounted on an engine for the idle exhaust route. The exhaust gases are led to the expansion chamber en route to the discharge port of the idle exhaust route and expanded before discharged to the atmosphere. Japanese Patent No. 2678920 also discloses an exhaust system that has an expansion chamber for the idle exhaust route. The expansion chamber is a cylindrically shaped member that is mounted on an outer surface of a driveshaft housing. The expansion chamber communicates with the main expansion route and also with the atmosphere via the idle exhaust port.
Idle exhaust noise can be reduced because of such expansion chambers; however, such a detachable chamber or a cylindrical member raises production costs. A need therefore exists for an improved exhaust system that can reduce cost for providing an expansion chamber within an idle exhaust route.